We recently conducted the first comprehensive, longitudinal study of ability grouping in the early elementary grades using the National Center for Education Statistics'Early Childhood Longitudinal Study-Kindergarten Class of 1998-99 (ECLS-K). We were impelled by equivocal evidence and heated scholarly debates over the significance of this widespread instructional practice. Our baseline investigation of "mapping tracking" in the early elementary grades revealed the extensive nature and complexities of homogeneous ability grouping at the onset of schooling. Investigating the consequences of such instructional practices for children's cognitive growth is the next logical step and thus our proposed project, "tracking" tracking and achievement growth. Our present study shows that students are homogeneously grouped for instruction in a variety of ways and at different organizational levels as early as kindergarten. The most common type of ability grouping occurs within-classroom, with two-thirds of US kindergartners being grouped for reading instruction. Placement of kindergartners into high, middle or low reading groups depends not only on their measured cognitive skills, but on their teachers'judgments and the character of their schools. Differences in placement and between-group achievement growth may contribute to emerging race/ethnic and gender achievement gaps. These gaps are initially small or explicable by differences in family resources, but become established by the third grade and constitute critical concerns of educational equity. To fully understand the impact of our current findings, we will develop analytical models that assess the contribution of within-class ability grouping to students'cognitive growth from kindergarten to the third grade. We will take into account student individual characteristics as well as the social and organizational characteristics of their schools. The ECLS-K data set coupled with multi-level modeling and propensity score matching makes this study possible. By considering interrelationships between within-class ability grouping and school organizational features, including other achievement grouping practices, we will inform theory and practice with respect to equalizing educational opportunities for children who enter school with varied backgrounds and cognitive skills.